Apparatus for detecting peaks of wavelength-division-multiplexed light, and apparatus for controlling said light

ABSTRACT

Disclosed is an apparatus for controlling wavelength-division-multiplexed light wherein overall power of wavelength-division-multiplexed light is rendered constant through control for uniformalizing the level of only one wave of maximum power, and wherein the levels of respective channels are made substantially uniform. Optical level control means controls the optical level of propagating wavelength-division-multiplexed light, and a portion of the wavelength-division-multiplexed light output from the output level control means is branched to a tunable optical filter, which selectively outputs the light of each wavelength contained in the wavelength-division-multiplexed light. The light of each wavelength output from the optical filter is photoelectrically converted to an electric signal by photoelectric conversion means. Peak detection means detects the maximum peak value of the electric signal output from the photoelectric conversion means, and a feedback signal is input to the optical level control means in such a manner that the maximum peak value becomes a set value.

BACKGROUND OF THE INVENTION

This invention relates to an apparatus for detecting the peaks ofwavelength-division-multiplexed light and an apparatus for controllingthis light. More particularly, the invention relates to a peak detectionapparatus for detecting the peaks of wavelength-division-multiplexedlight and a control apparatus for controlling the intensity ofwavelength-division-multiplexed light based upon maximum peak value.

The development of multimedia networks in recent years has beenaccompanied by increasing demand for information, and much greatercapacity and more flexible network formation are being sought fortrunk-line optical transmission systems where information isconcentrated. At the present time, transmission using wavelengthdivision multiplexing (WDM) is the most effective means of coping withsystem demand and has been implemented commercially mainly in NorthAmerica. In such WDM transmission systems, management of the opticallevel of each channel in the transmission line is important, and theadvantages and disadvantages of such management affect greatly theoperating state of such functional devices as optical amplifiers, andthis in turn changes transmission quality significantly. In WDMtransmission, therefore, management of each wavelength level and S/Nratio, etc., is ultimately required in all repeater stages.

However, deploying a level detector or level controller for eachwavelength at all repeater stages is inappropriate from the standpointof reducing the cost of optical transmission systems. There is demandfor the minimum necessary functions for detecting and controllingwavelength-division-multiplexed light in a simpler manner that takescost into consideration. Various measures have been proposed heretoforefrom this point of view.

FIG. 17 shows an example of an apparatus for detectingwavelength-division-multiplexed light according to the prior art. Here asimple light-spectrum monitor is provided. This is an example reportedin a paper (ECOC′ 97, Tu3, p. 147) by K. Otsuka et al. This is atechnique through which wavelength-division-multiplexed light emittedfrom a fiber 101 undergoes wavelength separation by a diffractiongrating 102 and impinges upon a photodiode array 103 so that the levelof each wavelength is detected by a photodiode. The levels of thewavelengths are detected by the minimum number of photodiodes necessaryfor point-to-point monitoring.

FIG. 18 illustrates a second example of an apparatus for detecting andcontrolling wavelength-division-multiplexed light according to the priorart. This example is reported in a paper (IEEE Photon. Tech. Lett., vol.10, p.734, 1998) by K. Suzuki et al. In this apparatus for detecting andcontrolling wavelength-division-multiplexed light, first and secondoptical-fiber amplifiers 110, 120 for control to uniformalize opticalgain are cascade-connected, a light attenuator 130 is provided betweenthem, and a feedback circuit 140 is provided in such a manner that theintensity of output light from the second optical-fiber amplifier 120will be rendered constant.

The first and second optical-fiber amplifiers 110, 120 respectivelyinclude rare-earth fibers, e.g., erbium-doped fibers 112, 122, foramplifying wavelength-division-multiplexed light; laser diodes(excitation light sources) 113, 123 for generating excitation light thewavelength of which is shorter than that of signal light and the energyof which is greater, and for introducing this light to the erbium-dopedfibers; optical branchers 114, 124; photoreceptors (photodiodes) 115,125 for detecting the power of the wavelength-division-multiplexed lightoutput by the respective optical-fiber amplifiers; photoreceptors(photodiodes) 116, 126 for detecting the power of thewavelength-division-multiplexed light input to the respectiveoptical-fiber amplifiers; and optical-gain controllers 117, 127 forinputting feedback signals to the excitation light sources 113, 123,respectively, of the respective optical-fiber amplifiers in such amanner that the power ratio (optical gain) of the input light of therespective optical-fiber amplifiers to the output light thereof willbecome a set gain.

The feedback circuit 140 includes a wavelength demultiplexer 141 forseparating the wavelength-division-multiplexed light, which is output bythe second optical-fiber amplifier, into individual wavelengths andoutputting the same; photodiodes 142 ₁-142 _(n) for detecting theintensities (levels) of respective wavelengths λ₁-λ_(n); a maximum-valuedetector 143 for detecting the maximum value from among the levels ofthe wavelengths; and an optical-output uniformalizing controller 144 forinputting a feedback signal to the light attenuator 130 in such a mannerthat the optimum value will become the set value. The light attenuator130 controls the optical level based upon the feedback signal.

In the second example of the prior art, a high output is obtained bycascade-connecting the optical-fiber amplifiers. In an optical-fiberamplifier, gain varies depending upon wavelength, though the gains ofthe respective wavelengths can be made uniform (thewavelength-dependence of gain can be uniformalized) by performingcontrol to uniformalize gain. Further, since the gains of respectivewavelengths can be made uniform, the levels of the respectivewavelengths can also be made approximately uniform. As a result, bydetecting the wavelength for which power is maximum and performingcontrol in such a manner that this maximum value becomes the set value,it becomes possible to perform control to uniformalize the power of theoutput light. In other words, it becomes possible to perform control touniformalize the power of the output light by controlling only one waveof the maximum power irrespective of the number of channels.

The second example of the prior art resembles the first example in thatlevel is detected on a per-wavelength basis. The second example of theprior art differs from the first example in that (1)wavelength-division-multiplexed light is separated into individualwavelengths, using a wavelength demultiplexer such as anarrayed-waveguide grating (AWG), in a state in which thewavelength-division-multiplexed light is enclosed within optical fiber;(2) after the power of each channel (each wave) is detected, the maximumvalue of these is calculated and fed back to the light attenuator touniformalize the power per channel; and (3) the number of wavelengths ofreceived light is limited owing to a limitation upon the number ofwavelength branches from the wavelength demultiplexer.

FIG. 19 shows a third example of an apparatus for detecting andcontrolling wavelength-division-multiplexed light according to the priorart. This is described in a report by Saeki, et al. (NEC Giho, vol. 51,no. 4, p. 45, 1998). In this apparatus for detecting and controllingwavelength-division-multiplexed light, a light circulator 150 inputsentrant wavelength-division-multiplexed light to a wavelengthdemultiplexer 160, which separates the wavelength-division-multiplexedlight into wavelengths λ₁-λ_(n) of respective channels. The thusdemultiplexed wavelengths λ₁-λ_(n) of the respective channels are inputto photodiodes 174 ₁-174 _(n) via variable light attenuators ¹⁷¹-171_(n), total reflection mirrors 172 ₁-172 _(n), and optical branchingcouplers 173 ₁-173 _(n), respectively. The photodiodes 174 ₁-174 _(n)photoelectrically convert the input light of the respective wavelengthsand feed back the resulting signals to the variable light attenuators¹⁷¹-171 _(n) . As a result, the levels of the respective wavelengths ofthe respective channels are regulated individually to a fixed level.

The apparatus for detecting wavelength-division-multiplexed light shownin FIG. 17 is structurally large in size and involves an optical system.Consequently, this arrangement is not suited to an apparatus fordetecting and controlling wavelength-division-multiplexed lightnecessary at all repeater stages.

The apparatus for detecting and controllingwavelength-division-multiplexed light according to the second example ofthe prior art in FIG. 18 requires a costly wavelength demultiplexer.Moreover, there is a limitation upon the number of channels into whichwavelength is divided by the wavelength demultiplexer, and the apparatuscannot deal flexibly with changes in the number of channels or changesin wavelength.

The apparatus for detecting and controllingwavelength-division-multiplexed light according to the third example ofthe prior art in FIG. 19 performs level adjustment individually for eachchannel and therefore makes possible control that is highly precise.However, the costly wavelength demultiplexer is required. Moreover, alight attenuator, total reflection mirror, optical brancher andphotodiode are required for each channel, as a result of which theapparatus takes on a large size. In addition, changes in the number ofchannels or changes in wavelength cannot be dealt with in a flexiblemanner.

Thus, as described above, a wide variety of methods and arrangementshave been proposed for detecting and controllingwavelength-division-multiplexed light, but each of these proposalsinvolves problems. The functions that are required of an apparatus fordetecting and controlling wavelength-division-multiplexed light are asfollows:

-   -   It should be possible to detect optical power per wave of light        whose wavelengths are allocated to respective channels, and to        perform control to uniformalize optical power (or to detect the        maximum value of optical power and perform control to        uniformalize the same.    -   It should be possible to ascertain the number of multiplexed        wavelengths.    -   The arrangement should be independent of channel wavelength and        number of multiplexed wavelengths.    -   The apparatus should be low in cost, small in size and simple in        structure.

The conventional examples of apparatus meet some but not all of theserequirements.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide anapparatus for detecting and an apparatus for controllingwavelength-division-multiplexed light in which it is possible, through asimple structure and without requiring a costly wavelengthdemultiplexer, and independently of the wavelength allocation of eachchannel (each wave) and number of multiplexed wavelengths, to controlthe power of output light to uniformalize the same by control of asingle wave for which power is maximum, and in which it is possible toascertain the number of multiplexed wavelengths.

According to the present invention, there is provided an apparatus fordetecting the peaks of wavelength-division-multiplexed light,comprising: (1) optical branching means for branching a portion ofwavelength-division-multiplexed light propagated; (2) a tunable opticalfilter for selectively outputting light of each wavelength of thebranched wavelength-division-multiplexed light; (3) photoelectricconversion means for photoelectrically converting light output from thetunable optical filter; and (4) peak detection means for detecting thepeaks of an electric signal output from the photoelectric conversionmeans.

In accordance with this apparatus for detectingwavelength-division-multiplexed light, a photodiode array and awavelength demultiplexer are not used, as a result of which theapparatus is small in size and simple in structure. Moreover, there isno limitation upon the number of channels (number of multiplexedwavelengths), and it is possible to detect maximum power from among thepowers of light of wavelengths contained in thewavelength-division-multiplexed light. Further, by providing periodicsweeping means, the light of each wavelength can be output periodicallyfrom the tunable optical filter, and it is possible to readily detectthe peak values/maximum peak value of power of the light of eachwavelength as well as the number of multiplexed wavelengths (number ofchannels). By cascade-connecting two or more tunable optical filters andperiodically sweeping these filters synchronously, light of a wavelengthhaving a narrow half-width can be output. This is advantageous in a casewhere wavelength spacing is small. By providing a light-equalizingfilter on the output side of the tunable optical filter, the precisionwith which the optical power of each wavelength is detected can beimproved and it is possible to improve the precision with which peakvalue and number of multiplexed wavelengths are detected.

An apparatus for controlling wavelength-division-multiplexed lightaccording to a first aspect of the present invention comprises: (1)optical level control means for controlling the optical level ofwavelength-division-multiplexed light propagated, or an optical-fiberamplifier for amplifying wavelength-division-multiplexed lightpropagated; (2) optical branching means for branching a portion ofwavelength-division-multiplexed light output from the optical levelcontrol means or optical-fiber amplifier; (3) a tunable optical filterfor selectively outputting light of each wavelength of the branchedwavelength-division-multiplexed light; (4) photoelectric conversionmeans for photoelectrically converting light output from the tunableoptical filter; (5) peak detection means for detecting the peaks of anelectric signal output from the photoelectric conversion means; and (6)feedback means for inputting a feedback signal to the optical levelcontrol means or an excitation light source of the optical-fiberamplifier in such a manner that maximum peak value will become a setvalue.

In accordance with the apparatus for controllingwavelength-division-multiplexed light of the first aspect of theinvention, a photodiode array and a wavelength demultiplexer are notused, as a result of which the apparatus is small in size and simple instructure. Moreover, it is possible to detect maximum power from amongthe powers of light of wavelengths contained in thewavelength-division-multiplexed light, with no limit upon the number ofchannels, and control can be carried out in such a manner that thismaximum value becomes the set value. As a result, the power of outputlight can be controlled so as to be uniformalized. By cascade-connectingtwo or more tunable optical filters and periodically sweeping thesefilters synchronously, light of a wavelength having a narrow half-widthcan be output. Even in a case where wavelength spacing is small, theprecision with which the optical level of each wavelength is detectedcan be improved, thereby making possible highly precise control foruniformalizing the level of output light. By providing alight-equalizing filter on the output side of the tunable opticalfilter, the precision with which the optical power of each wavelength isdetected is improved and it is possible to perform highly precisecontrol to uniformalize the level of output light.

Further, in dependence upon the maximum peak value, the feedback means(1) produces a feedback signal in such a manner that the maximum peakvalue will become the set value, or (2) produces a feedback value insuch a manner that total detected power of thewavelength-division-multiplexed light becomes the set power, and inputsthe feedback signal to the excitation light source of the optical-fiberamplifier. If this arrangement is adopted, control for uniformalizingthe maximum value can be performed effectively even in a case where themaximum peak value exceeds the optical level of each wavelength.Further, the number of multiplexed wavelengths is detected based uponthe number of peaks of the electric signal output from the photoelectricconversion means, and the set power is changed in conformity with thenumber of multiplexed wavelengths, thereby making possible excellentcontrol for uniformalizing optical level.

Further, in dependence upon the maximum peak value, the feedback means(1) produces a feedback signal in such a manner that the maximum peakvalue will become the set value, or (2) produces a feedback value insuch a manner that detected gain (power ratio of the input light of theoptical-fiber amplifier to the output light thereof) becomes the setgain, and inputs the feedback signal to the excitation light source ofthe optical-fiber amplifier. If this arrangement is adopted, excessivegain tilt will not be caused and control for uniformalizing maximumvalue can be performed effectively.

An apparatus for controlling wavelength-division-multiplexed lightaccording to a second aspect of the present invention comprises: (1) anoptical-fiber amplifier for amplifying wavelength-division-multiplexedlight propagated; (2) feedback means for inputting a feedback signal toan excitation light source of the optical-fiber amplifier in such amanner that optical gain, which is a power ratio of the input light ofthe optical-fiber amplifier to the output light thereof, becomes a setgain; (3) optical level control means for controlling the optical levelof wavelength-division-multiplexed light output from the optical-fiberamplifier; (4) optical branching means for branching a portion ofwavelength-division-multiplexed light output from the optical levelcontrol means; (5) a tunable optical filter for selectively outputtinglight of each wavelength of the branched wavelength-division-multiplexedlight; (6) photoelectric conversion means for photoelectricallyconverting light output from the tunable optical filter; (7) peakdetection means for detecting the peaks of an electric signal outputfrom the photoelectric conversion means; and (8) feedback means forinputting a feedback signal to the optical level control means in such amanner that the maximum peak value will become a set value; the opticallevel control means performing optical level control based upon thefeedback signal.

In accordance with the apparatus for controllingwavelength-division-multiplexed light of the second aspect of theinvention, control for uniformalizing gain is performed in a gainuniformalizing controller, thereby uniformalizing the gain of eachchannel. As a result, control for uniformalizing maximum value isperformed in a state in which the level of the light of each wavelengthis approximately uniformalized. It therefore becomes possible to performcontrol for uniformalizing the power of output light by controlling onlyone wave of the maximum power without relation to the number ofchannels. Moreover, the level of each channel can be made uniform.

An apparatus for controlling wavelength-division-multiplexed lightaccording to a third aspect of the present invention comprises: (1) afirst optical-fiber amplifier for amplifyingwavelength-division-multiplexed light propagated; (2) first feedbackmeans for inputting a feedback signal to an excitation light source ofthe first optical-fiber amplifier in such a manner that optical gain,which is a power ratio of the input light of the first optical-fiberamplifier to the output light thereof, becomes a set gain; (3) opticallevel control means for controlling the optical level ofwavelength-division-multiplexed light output from the firstoptical-fiber amplifier; (4) a second optical fiber amplifier foramplifying wavelength-division-multiplexed light output from the opticallevel control means; (5) second feedback means for inputting a feedbacksignal to an excitation light source of the second optical-fiberamplifier in such a manner that optical gain, which is a power ratio ofthe input light of the second optical-fiber amplifier to the outputlight thereof, becomes a set gain; (6) optical branching means forbranching a portion of wavelength-division-multiplexed light output fromthe second optical-fiber amplifier; (7) a tunable optical filter forselectively outputting light of each wavelength of the branchedwavelength-division-multiplexed light; (8) photoelectric conversionmeans for photoelectrically converting light output from the tunableoptical filter; (9) peak detection means for detecting the peaks of anelectric signal output from the photoelectric conversion means; and (10)third feedback means for inputting a feedback signal to the opticallevel control means in such a manner that the maximum peak value willbecome a set value; the optical level control means performing opticallevel control based upon the feedback signal.

In accordance with the apparatus for controllingwavelength-division-multiplexed light of the third aspect of theinvention, it is possible to perform control for uniformalizing thepower of output light by controlling only one wave of the maximum powerwithout relation to the number of channels. Moreover, the level of eachchannel can be made uniform. Furthermore, it is possible to achieve ahigh output and multiple channels.

Other features and advantages of the present invention will be apparentfrom the following description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a first embodiment of an apparatus for detecting thepeaks of wavelength-division-multiplexed light according to the presentinvention;

FIG. 2 is a diagram showing the construction of a tunable opticalfilter;

FIG. 3 is a diagram showing the tuning characteristic of the tunableoptical filter;

FIG. 4 is a diagram useful in describing an incidence spectrum ofwavelength-division-multiplexed light and a change in the level ofreceived light with time;

FIG. 5 illustrates a second embodiment of an apparatus for detecting thepeaks of wavelength-division-multiplexed light according to the presentinvention;

FIG. 6 illustrates a third embodiment of an apparatus for detecting thepeaks of wavelength-division-multiplexed light according to the presentinvention;

FIG. 7 illustrates a first embodiment of an apparatus for controllingwavelength-division-multiplexed light according to the presentinvention;

FIG. 8 illustrates a second embodiment of an apparatus for controllingwavelength-division-multiplexed light according to the presentinvention;

FIG. 9 illustrates a third embodiment of an apparatus for controllingwavelength-division-multiplexed light according to the presentinvention;

FIG. 10 is a diagram showing the construction of a control correctionunit in FIG. 9;

FIG. 11 illustrates a modification of the third embodiment;

FIG. 12 illustrates a fourth embodiment of an apparatus for controllingwavelength-division-multiplexed light according to the presentinvention;

FIG. 13 is a diagram showing the construction of a control correctionunit in FIG. 12;

FIG. 14 illustrates a fifth embodiment of an apparatus for controllingwavelength-division-multiplexed light according to the presentinvention;

FIG. 15 illustrates a sixth embodiment of an apparatus for controllingwavelength-division-multiplexed light according to the presentinvention;

FIG. 16 illustrates a seventh embodiment of an apparatus for controllingwavelength-division-multiplexed light according to the presentinvention;

FIG. 17 is a diagram useful in describing an apparatus for detectingwavelength-division-multiplexed light according to the prior art;

FIG. 18 is a diagram showing the construction of an apparatus fordetecting/controlling wavelength-division-multiplexed light according tothe prior art; and

FIG. 19 is a diagram showing the construction of another example of anapparatus for detecting/controlling wavelength-division-multiplexedlight according to the prior art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS (A) Apparatus for Detectingwavelength-division-multiplexed Light

First Embodiment

FIG. 1 is a diagram showing the construction of a first embodiment of anapparatus for detecting the peaks of wavelength-division-multiplexedlight according to the present invention. The apparatus includes anoptical fiber 11 through which wavelength-division-multiplexed lightpropagates; an optical branching coupler 12 for branching thewavelength-division-multiplexed light; a tunable optical filter 13, thecenter wavelength of which is variable, for selectively outputting lightof each wavelength of the branched wavelength-division-multiplexedlight; a photodiode (PD) 14 serving as photoelectric conversion meansfor photoelectrically converting light output from the tunable opticalfilter 13; and a peak detection circuit 15 for detecting the peak (e.g.,maximum peak) of the electric signal output from the photodiode 14.

Examples of the tunable optical filter 13 are an acousto-optic tunableoptical filter, an electro-optic tunable optical filter, a thermo-optictunable optical filter, and a mechanical tunable optical filter.According to the present invention, however, the acousto-optic tunableoptical filter or electro-optic tunable optical filter is ideal from thestandpoint of sweep speed.

FIG. 2 is a diagram showing the construction of an acousto-optic tunableoptical filter. The filter includes a SAW waveguide 13 a formed on asubstrate which exhibits an electro-optic effect, such as a substratemade of LiNbO₃ (lithium niobate); an interdigital transducer (IDT) 13 b;a SAW clad 13 c formed by diffusing titanium so as to clad the SAWwaveguide; absorbers 13 d, 13 e for absorbing surface acoustic waves(SAW); optical waveguides 13 f, 13 g formed by diffusing titanium;crossed polarization beam splitters (PBS) 13 h, 13 i, which are arrangedto embrace two linear waveguides, for operating independently ofpolarization; and a high-frequency signal application unit 13 j forapplying a high-frequency signal of 170-180 MHz to the interdigitaltransducer 13 b . The high-frequency signal application unit 13 jincludes a high-frequency generator 13 j-1 and an inductance 13 j-2,which is for canceling the input capacitance of the interdigitaltransducer 13 b, connected in series with the high-frequency generator13 j-1. When the high-frequency signal is applied to the interdigitaltransducer 13 b, a surface acoustic wave is generated, which has theeffect of rotating, by 90°, the polarization of a specific wavelengthconforming to this frequency. Accordingly, the polarization beamsplitters 13 h, 13 i are provided on the input and output sides,respectively, to split the polarization, thereby making it possible torealize a tunable optical filter. For example, ifwavelength-division-multiplexed light of the TE mode is input to thetunable optical filter 13 as input light, only the polarization of thewavelength corresponding to the frequency of the high-frequency signalapplied to the interdigital transducer is rotated 90°, whereby TM-modepolarization is obtained. The TM-mode polarized light is output from theoptical waveguide 13 g.

FIG. 3 shows the tuning characteristic of the tunable optical filter.The frequency of the high-frequency signal is plotted along thehorizontal axis and the selected wavelength along the vertical axis. Theselected wavelength shortens in inverse proportion to the frequency ofthe high-frequency signal.

Accordingly, the tunable optical filter 13 is capable of selectivelyoutputting, in successive fashion, the wavelengths contained in theinput light by sweeping, at prescribed cycles, the frequency of thehigh-frequency signal output from the high-frequency signal applicationunit 13 j.

Thus, when wavelength-division-multiplexed light enters the opticalfiber 11, the optical branching coupler 12 branches a part of thewavelength-division-multiplexed light and inputs this light to thetunable optical filter 13. The latter sweeps the center wavelengthperiodically at prescribed cycles. As a result, the light of eachwavelength contained in the wavelength-division-multiplexed light isseparated successively and input to the photodiode 14. The latterphotoelectrically converts the input light to an electric signal andapplies the electric signal to the peak detection circuit 15.

FIG. 4 is a diagram useful in describing an incidence spectrum ofwavelength-division-multiplexed light and a change in the level ofreceived light with time. Since the tunable optical filter 13 sweeps thecenter wavelength periodically at prescribed cycles, the optical powerof the light received by the photodiode 14 indicates a change in powerin which the wavelength axis is converted to the time axis. For example,if the incident light is wavelength-division-multiplexed light in whichlight of wavelengths λ₁, λ₂, λ₄ and λ₅ shown in FIG. 4(a) has beenmultiplexed, the change in level after the photoelectric conversion willbe as shown in FIG. 4(b) owing to the sweeping of the center wavelengthback and forth. The peak detection circuit 15 subjects the waveform ofFIG. 4(b) to peak-value detection and detects the maximum peak value oreach peak value and the number of peaks. The maximum peak valueindicates the optical level (optical power) of the wavelength of lightfor which the spectrum is maximum, this light being detected from thelight of a number of wavelengths contained in thewavelength-division-multiplexed light.

(b) Second Embodiment

FIG. 5 is a diagram showing the construction of a second embodiment ofan apparatus for detecting the peaks of wavelength-division-multiplexedlight according to the present invention. Components in FIG. 5 identicalwith those of the first embodiment shown in FIG. 1 are designated bylike reference characters. The second embodiment differs from the firstembodiment in that two or more tunable optical filters 13 ₁-13 _(n), areconnected in cascade, each of the tunable optical filters 13 ₁-13 _(n)being swept synchronously. With only one tunable optical filter,wavelength width for which the peak value is halved (i.e., thehalf-width) broadens and wavelength selectivity deteriorates. If aplurality of tunable optical filters 13 ₁-13 _(n) are connected incascade, as in the second embodiment, it is possible to output light ofa wavelength having a narrower half-width and to improve wavelengthselectivity.

(c) Third Embodiment

FIG. 6 is a diagram showing the construction of a third embodiment of anapparatus for detecting the peaks of wavelength-division-multiplexedlight according to the present invention. Components in FIG. 6 identicalwith those of the first embodiment shown in FIG. 1 are designated bylike reference characters. The third embodiment differs from the firstembodiment in that a light-equalizing filter 16 is provided between thetunable optical filter 13 and photodiode 14. Even if the spectrums ofthe wavelengths are identical, the peak values of the wavelengths outputfrom the tunable optical filter 13 differ and accurate peak detectioncannot be carried out. In the third embodiment, therefore, the outputside of the tunable optical filter 13 is provided with thelight-equalizing filter 16 having an equalization characteristic suchthat if the input spectrums are identical, the peak values of therespective wavelengths will take on the same level. As a result, peakdetection and detection of maximum peak value can be performed with goodprecision.

(B) Apparatus for Controlling Wavelength-division-multiplexed Light

(a) First Embodiment

FIG. 7 is a diagram showing the construction of a first embodiment of anapparatus for controlling wavelength-division-multiplexed lightaccording to the present invention. The apparatus includes an opticalfiber 21 through which wavelength-division-multiplexed light propagates;an optical branching coupler 22 for branching thewavelength-division-multiplexed light; a wavelength-division-multiplexedlight peak detector 23 for detecting the peaks ofwavelength-division-multiplexed light; and a device 24, such as avariable light attenuator, for controlling the optical level of outputlight. Examples of the device for controlling the optical level ofoutput light are, in addition to the variable light attenuator, anexternal optical modulator and a semiconductor optical amplifier.

The wavelength-division-multiplexed light peak detector 23 has astructure identical with that of the apparatus for detecting the peaksof wavelength-division-multiplexed light shown in FIG. 1. Specifically,the detector 23 includes the tunable optical filter 13, the centerwavelength of which is variable, for selectively outputting light ofeach wavelength of the branched wavelength-division-multiplexed light;the photodiode 14 serving as photoelectric conversion means forphotoelectrically converting light output from the tunable opticalfilter 13; and the peak detection circuit 15 for detecting the maximumpeak value of the electric signal output from the photodiode 14 andinputting this value to the variable light attenuator 24.

When wavelength-division-multiplexed light enters the optical fiber 21,the optical branching coupler 22 branches a part of thewavelength-division-multiplexed light output from the variable lightattenuator 24 and inputs this light to the tunable optical filter 13 ofthe wavelength-division-multiplexed light peak detector 23. The lattersweeps the center wavelength periodically at prescribed cycles. As aresult, the light of each wavelength contained in thewavelength-division-multiplexed light is separated successively andinput to the photodiode 14. The latter photoelectrically converts theinput light to an electric signal and applies the electric signal to thepeak detection circuit 15. The latter detects the maximum peak value,namely the optical level of the wavelength of light for which thespectrum is maximum, this light being detected from the light of anumber of wavelengths contained in the wavelength-division-multiplexedlight. The peak detection circuit 15 generates a feedback signal in sucha manner that the maximum peak value becomes the set value and inputsthis feedback signal to the variable light attenuator 24. For example,the difference between the detected maximum peak value and the set valueis input to the variable light attenuator 24 as the feedback signal. Thevariable light attenuator 24 controls the level of the output lightbased upon the feedback signal. The above-described feedback control isthenceforth performed continuously so that the maximum peak value willbecome the set value.

It should be noted that an arrangement can be adopted in which theoutput side of the peak detection circuit 15 is provided with thelight-equalizing filter 16 for calculating the difference between themaximum peak value and the set value and inputting the difference to thevariable light attenuator 24.

To sum up the foregoing, there is obtained a temporal change in thelevel of received light corresponding to the spectrum of the light ofeach wavelength contained in the wavelength-division-multiplexed light,peak detection is carried out to detect the maximum value of the channel(the maximum peak value), and the device that controls the optical levelis subjected to feedback control based upon this detected value.

If gain equalization with regard to each wavelength is performedsatisfactorily, then the level error of each channel (the light of eachwavelength) may be considered to be small. Further, the largest value ofthe output level per channel is decided mainly by the non-linearity ofthe optical fiber constituting the transmission line. Accordingly,control for satisfactory uniformalization of level over all channels canbe carried out by detection of maximum value and control to uniformalizemaximum value in the manner described above.

The foregoing relates to a case where the arrangement of FIG. 1 is usedas the wavelength-division-multiplexed light peak detector 23. However,it is also possible to adopt the arrangement of FIG. 5 in which thetunable optical filters 13 ₁-13 _(n) are cascade-connected, or thearrangement of FIG. 6 in which the light-equalizing filter 16 isprovided on the output side of the tunable optical filter 13.

(b) Second Embodiment

FIG. 8 is a diagram showing the construction of a third embodiment of anapparatus for controlling wavelength-division-multiplexed lightaccording to the present invention. Components in FIG. 8 identical withthose of the first embodiment shown in FIG. 7 are designated by likereference characters. The second embodiment shown in FIG. 8 differs fromthe first embodiment in that an optical-fiber amplifier is used insteadof the variable light attenuator as means for controlling the level ofthe output light.

As shown in FIG. 8, there are provided optical isolators 25, 26; awavelength multiplexing coupler 27 for multiplexing excitation light andsignal light; an optical amplifying fiber 28, such as erbium-doped fiber(EDF), for amplifying signal light; and a laser diode (excitation lightsource) 29 for generating excitation wavelength light the wavelength ofwhich is shorter than that of the signal light but the energy of whichis greater, and inputting this light to the optical amplifying fiber 28.

Wavelength-division-multiplexed light (signal light) that has enteredthe optical fiber 21 passes through the optical isolator 25 and ismultiplexed with the excitation wavelength light, which is emitted bythe excitation light source 29, in the wavelength multiplexing coupler27. The resulting light enters the optical amplifying fiber 28 and isamplified. The amplified wavelength-division-multiplexed light passesthrough the optical isolator 26 and reaches the optical branchingcoupler 22. The latter branches a part of thewavelength-division-multiplexed light and inputs this light to thetunable optical filter 13 of the wavelength-division-multiplexed lightpeak detector 23. The tunable optical filter 13 sweeps the centerwavelength periodically at prescribed cycles. As a result, the light ofeach wavelength contained in the wavelength-division-multiplexed lightis separated successively and input to the photodiode 14. The latterphotoelectrically converts the power of the light of each wavelength toan electric signal and applies the electric signal to the peak detectioncircuit 15. The latter detects the maximum peak value, namely theoptical level of the wavelength of light for which the spectrum ismaximum, this light being detected from the light of a number ofwavelengths contained in the wavelength-division-multiplexed light. Thepeak detection circuit 15 generates a feedback signal in such a mannerthat the maximum peak value becomes the set value and inputs thisfeedback signal to the excitation light source 29. The latter controlsthe intensity of the excitation wavelength based upon the feedbacksignal and controls the level of the light output from opticalamplifying fiber 28. The above-described feedback control is thenceforthperformed continuously so that the maximum peak value will become theset value.

It should be noted that an arrangement can be adopted in which theoutput side of the peak detection circuit 15 is provided with a feedbackcircuit 20 for calculating the difference between the maximum peak valueand the set value and inputting the difference to the excitation lightsource 29.

In maximum-value detection and control for uniformalizing maximum valueaccording to the second embodiment, control for satisfactoryuniformalization of level over all channels can be carried out forreasons the same as those set forth in the description of the firstembodiment.

The foregoing relates to a case where the arrangement of FIG. 1 is usedas the wavelength-division-multiplexed light peak detector 23. However,it is also possible to adopt the arrangement of FIG. 5 in which thetunable optical filters 13 ₁-13 _(n) are cascade-connected, or thearrangement of FIG. 6 in which the light-equalizing filter 16 isprovided on the output side of the tunable optical filter 13.

(c) Third Embodiment

FIG. 9 is a diagram showing the construction of a third embodiment of anapparatus for controlling wavelength-division-multiplexed lightaccording to the present invention. Components in FIG. 9 identical withthose of the second embodiment shown in FIG. 8 are designated by likereference characters. The third embodiment shown in FIG. 9 differs fromthe second embodiment in that (1) a control loop for uniformalizingmaximum value and a control loop for uniformalizing total power areprovided as means for controlling the level of output light, and (2)control for uniformalizing maximum value and control for uniformalizingtotal power is performed appropriately based upon the difference betweenthe maximum peak value and set peak value.

According to the first and second embodiments, control is carried out onthe assumption that the level error of each channel (the light of eachwavelength) is small. However, depending upon conditions, there arecases where the maximum peak value (the peak value of the light of acertain wavelength) becomes too large in comparison with the peak valuesof the light of the other wavelengths. In such cases the control of thefirst and second embodiments for uniformalizing maximum value is suchthat the total power is dominated by the power (the maximum peak value)of the light of the wavelength for which the excessive value has beenattained. As a result, the light output cannot be uniformalized and thelevel difference between channels grows. Accordingly, if the maximumpeak value becomes excessive, control for uniformalizing the total powerof the output light is carried out to reduce the level differencebetween channels. If the maximum peak value is not excessive, controlfor uniformalizing maximum value is carried out to uniformalize theoutput light and level is made approximately uniform overall allchannels. In other words, ifP ₀ <<P _(peak) ·Nholds (where P₀ represents total power and P_(peak) represents thedetected value of the peak), then control for uniformalizing total poweris performed and feedback based upon total power P₀ is made dominant.

The arrangement of FIG. 9 is provided with a branching coupler 30 forfurther branching the wavelength-division-multiplexed light branched bythe optical branching coupler 22 and inputting the branched light to thecontrol loop for uniformalizing maximum value and the control loop foruniformalizing total power; a photodiode 31 for photoelectricallyconverting the wavelength-division-multiplexed light to an electricsignal; a total-power uniformalizing controller 32 for detecting totalpower of the wavelength-division-multiplexed light (output light) fromthe input electric signal and outputting the difference between thedetected value of power and a set value of power; and a controlcorrection unit 33 for appropriately inputting a feedback signal to theexcitation light source 29 of the optical-fiber amplifier in such amanner that (1) the detected value of the peak becomes the set value ofthe peak or (2) the detected value of power becomes the set value ofpower, depending upon the difference between the detected maximum peak(detected value of the peak) and the set value of the peak.

FIG. 10 illustrates an example of the construction of the controlcorrection unit 33, which includes an operational amplifier 33 a forcalculating and outputting the difference between the detected value ofthe peak and the set value of the peak; an operational amplifier 33 bfor calculating and outputting the difference between the detected valueof power and the set value of power; and diodes 33 c, 33 e constructingdiode switches and connected so as to deliver the output of whichever ofthe two operational amplifiers exhibits the higher level.

If the detected value of a peak becomes significantly greater than theset value of a peak, the output of the operational amplifier 33 a isnegative and the absolute value thereof increases. Even if the detectedvalue of the peak increases, the variation in detected power is smalland so is the absolute value of the output of the operational amplifier33 b . As a result, the output level of the operational amplifier 33 bexceeds the output level of the operational amplifier 33 a , thedifference between the detected value of power and the set value ofpower is input to the excitation light source 29 as a feedback signaland control is performed so as to reduce this power difference to zero.

If the difference between the detected value of the peak and the setvalue of the peak is small, on the other hand, the output level of theoperational amplifier 33 a will exceed the output level of theoperational amplifier 33 b , the difference between the detected valueof the peak and the set value of the peak is input to the excitationlight source 29 as the feedback value and control is performed in such amanner that the detected value of the peak will become the set value ofthe peak.

The third embodiment described above relates to a case where thearrangement of FIG. 1 is used as the wavelength-division-multiplexedlight peak detector 23. However, it is also possible to adopt thearrangement of FIG. 5 in which the tunable optical filters 13 ₁-13 _(n)are cascade-connected, or the arrangement of FIG. 6 in which thelight-equalizing filter 16 is provided on the output side of the tunableoptical filter 13.

Further, according to the third embodiment, the number N of channels isalready known and the set value of power conforming to this number ofchannels is fixed. However, an arrangement can be adopted in which thenumber of channels is detected and the set value of power is decided independence upon the number of channels detected. FIG. 11 shows amodification of the third embodiment, which includes a wavelengthcounter 34. By counting the peaks in a waveform of the kind shown inFIG. 4(b), a count of the light of the wavelengths contained in thewavelength-division-multiplexed light is obtained. More specifically,when the output signal of the photodiode 14 exceeds a predeterminedthreshold value, “1” is recognized at the TTL level, counting isperformed at the rising edge thereof, and the value of the countrecorded in the sweeping half-period of the tunable optical filter 13 isinput to the total-power uniformalizing controller 32 as the count.

(d) Fourth Embodiment

FIG. 12 is a diagram showing the construction of a fourth embodiment ofan apparatus for controlling wavelength-division-multiplexed lightaccording to the present invention. Components in FIG. 12 identical withthose of the second embodiment shown in FIG. 8 are designated by likereference characters. The fourth embodiment shown in FIG. 12 differsfrom the second embodiment in that (1) a control loop for uniformalizingmaximum value and a control loop for uniformalizing gain are provided asmeans for controlling the level of output light, and (2) control foruniformalizing maximum value and control for uniformalizing gain isperformed appropriately based upon the difference between the maximumpeak value and set peak value.

According to the first and second embodiments, control is carried out onthe assumption that the level error of each channel (the light of eachwavelength) is small. However, depending upon conditions, there arecases where the maximum peak value (the peak value of the light of acertain wavelength) becomes too large in comparison with the peak valuesof the light of the other wavelengths. In such cases the control of thefirst and second embodiments for uniformalizing maximum value is suchthat the total power is dominated by the power (the maximum peak value)of the light of the wavelength for which the excessive value has beenattained. As a result, the light output cannot be uniformalized and thelevel difference between channels grows. Accordingly, if the maximumpeak value becomes excessive, control for uniformalizing gain is carriedout to approximately uniformalize the gains of the respective channels.In other words, with an optical-fiber amplifier, gain varies independence upon wavelength. By performing control to uniformalize gain,however, the gains of the respective wavelengths can be made uniform(the dependence of gain upon wavelength can be uniformalized). If thegains of respective wavelengths become uniform, the level differencebetween channels becomes small. If the maximum peak value is no longerexcessive as a result, control for uniformalizing the maximum value iscarried to make the levels across all channels approximately uniform. Ifsuch an expedient is adopted, control for uniformalizing maximum valuecan be performed effectively by monitoring optical gain without causingexcessive gain tilt with respect to wavelength.

The arrangement of FIG. 12 is provided with the branching coupler 30 forfurther branching the output light (the wavelength-division-multiplexedlight) branched by the optical branching coupler 22 and inputting thebranched light to the control loop for uniformalizing maximum value andthe control loop for uniformalizing gain; a photodiode 31 forphotoelectrically converting the output light, which has been branchedby the branching coupler 30, to an electric signal; an optical branchingcoupler 41 for branching the input light (thewavelength-division-multiplexed light); a photodiode 42 forphotoelectrically converting input light, which has been branched by thebranching coupler 41, to an electric signal; a gain uniformalizingcontroller 43 for obtaining output power and input power based upon theelectric signals output from the photodiodes 31, 42, calculating opticalgain from the ratio between these and outputting a signal conforming tothe difference between detected gain and set gain; and a controlcorrection unit 44 for appropriately inputting a feedback signal to theexcitation light source 29 of the optical-fiber amplifier in such amanner that (1) the detected value of the peak becomes the set value ofthe peak or (2) the detected value of optical gain becomes the set valueof optical gain, depending upon the difference between the detectedmaximum peak (detected value of the peak) and the set value of the peak.

FIG. 13 illustrates an example of the construction of the controlcorrection unit 44, which includes an operational amplifier 44 a forcalculating and outputting the difference between the detected value ofthe peak and the set value of the peak; an operational amplifier 44 bfor calculating and outputting the difference between the detected valueof optical gain and the set value of optical gain; and diodes 44 c, 44 econstructing diode switches and connected so as to deliver the output ofwhichever of the two operational amplifiers exhibits the higher level.

If the detected value of a peak becomes significantly greater than theset value of a peak, the output of the operational amplifier 44 a isnegative and the absolute value thereof increases. Even if the detectedvalue of the peak increases, the variation in detected gain is small andso is the absolute value of the output of the operational amplifier 44 b. As a result, the output level of the operational amplifier 44 bexceeds the output level of the operational amplifier 44 a , thedifference between the detected value of optical gain and the set valueof optical gain is input to the excitation light source 29 as a feedbacksignal and control is performed in such a manner that optical gain willbe uniformalized.

If the difference between the detected value of the peak and the setvalue of the peak is small, on the other hand, the output level of theoperational amplifier 44 a will exceed the output level of theoperational amplifier 44 b, the difference between the detected value ofthe peak and the set value of the peak is input to the excitation lightsource 29 as the feedback value and control is performed in such amanner that the detected value of the peak will become the set value ofthe peak.

The fourth embodiment described above relates to a case where thearrangement of FIG. 1 is used as the wavelength-division-multiplexedlight peak detector 23. However, it is also possible to adopt thearrangement of FIG. 5 in which the tunable optical filters 13 ₁-13 _(n)are cascade-connected, or the arrangement of FIG. 6 in which thelight-equalizing filter 16 is provided on the output side of the tunableoptical filter 13.

(e) Fifth Embodiment

FIG. 14 is a diagram showing the construction of a fifth embodiment ofan apparatus for controlling wavelength-division-multiplexed lightaccording to the present invention. This is an example in which anoptical-fiber amplifier for control to uniformalize gain is provided onthe input side of the apparatus for controllingwavelength-division-multiplexed light according to the first embodiment(FIG. 7), and control for uniformalizing gain is performed independentlyof control for uniformalizing maximum value.

The arrangement of FIG. 14 includes a maximum-value uniformalizingcontroller 20 and an optical-gain uniformalizing controller 50. Themaximum-value uniformalizing controller 20, which is for uniformalizingmaximum peak value, has a structure identical with that of the apparatusfor controlling wavelength-division-multiplexed light according to thefirst embodiment (FIG. 7). This includes the optical fiber 21 throughwhich wavelength-division-multiplexed light propagates; the opticalbranching coupler 22 for branching the wavelength-division-multiplexedlight; the wavelength-division-multiplexed light peak detector 23 fordetecting the peaks of wavelength-division-multiplexed light; and thevariable light attenuator 24 for controlling the optical level of outputlight. The wavelength-division-multiplexed light peak detector 23includes the tunable optical filter 13, the photodiode 14 serving asphotoelectric conversion means, and the peak detection circuit 15.

The optical-gain uniformalizing controller 50, which is foruniformalizing the ratio between the output level of the optical-fiberamplifier and the input level thereto (i.e., the gain), includes anoptical fiber 51; optical isolators 52, 53; a wavelength multiplexingcoupler 54 for multiplexing excitation light and signal light; anoptical amplifying fiber 55, such as erbium-doped fiber (EDF), foramplifying signal light; a laser diode (excitation light source) 56 forgenerating excitation wavelength light the wavelength of which isshorter than that of the signal light but the energy of which isgreater, and inputting this light to the optical amplifying fiber 55; abranching coupler 57 for branching the output light(wavelength-division-multiplexed light) of the optical-fiber amplifier;a photodiode 58 for photoelectrically converting the output light, whichhas been branched by the branching coupler, to an electric signal; anoptical branching coupler 59 for branching input light(wavelength-division-multiplexed light); a photodiode 60 forphotoelectrically converting the input light, which has been branched bythe branching coupler, to an electric signal; and a gain uniformalizingcontroller 61 for obtaining power of the output light and power of theinput light based upon the electric signals output from the photodiodes58, 60, detecting optical gain from the ratio between these andinputting a feedback signal, which conforms to the difference betweendetected gain and set gain, to the excitation light source 56.

In accordance with the fifth embodiment, control for uniformalizing gainis performed in the optical-gain uniformalizing controller 50, therebymaking it possible to make the gains of the respective channels uniform.As a result, the levels of the respective channels can be madeapproximately uniform. Control for uniformalizing maximum value isperformed in the maximum-value uniformalizing controller 20, therebymaking it possible to perform control to uniformalize the power of theoutput light through control of only a single wave of maximum power withrelation to the number of channels. Moreover, the levels (powers) of therespective channels can be made uniform.

Further, in accordance with the fifth embodiment, control to maintaingain tilt and control of optical level per channel can be madeindependent of each other, and it is possible to avoid the combining ofcontrol and an increase in the complexity of control.

The fifth embodiment described above relates to a case where thearrangement of FIG. 1 is used as the wavelength-division-multiplexedlight peak detector 23. However, it is also possible to adopt thearrangement of FIG. 5 in which the tunable optical filters 131 ₁-13 _(n)are cascade-connected, or the arrangement of FIG. 6 in which thelight-equalizing filter 16 is provided on the output side of the tunableoptical filter 13.

(f) Sixth Embodiment

FIG. 15 is a diagram showing the construction of a sixth embodiment ofan apparatus for controlling wavelength-division-multiplexed lightaccording to the present invention. Components in FIG. 15 identical withthose of the fifth embodiment shown in FIG. 14 are designated by likereference characters. The sixth embodiment provides a secondoptical-gain uniformalizing controller 70 within the maximum-valueuniformalizing controller 20 of the fifth embodiment. Bycascade-connecting the optical-gain uniformalizing controllers 50 and70, a high output is obtained.

As shown in FIG. 15, the second optical-gain uniformalizing controller70 has a structure identical with that of the first optical-gainuniformalizing controller 50 and includes an optical fiber 71; opticalisolators 72, 73; a wavelength multiplexing coupler 74 for multiplexingexcitation light and signal light; an optical amplifying fiber 75, suchas erbium-doped fiber (EDF), for amplifying signal light; a laser diode(excitation light source) 76 for generating excitation wavelength lightthe wavelength of which is shorter than that of the signal light but theenergy of which is greater, and inputting this light to the opticalamplifying fiber 75; a branching coupler 77 for branching the outputlight (wavelength-division-multiplexed light) of the optical-fiberamplifier; a photodiode 78 for photoelectrically converting the outputlight, which has been branched by the branching coupler, to an electricsignal; an optical branching coupler 79 for branching input light(wavelength-division-multiplexed light); a photodiode 80 forphotoelectrically converting the input light, which has been branched bythe branching coupler, to an electric signal; and a gain uniformalizingcontroller 81 for obtaining power of the output light and power of theinput light based upon the electric signals output from the photodiodes78, 80, detecting optical gain from the ratio between these andinputting a feedback signal, which conforms to the difference betweendetected gain and set gain, to the excitation light source 76.

In accordance with the sixth embodiment, control for uniformalizing gainis performed in the gain uniformalizing controllers 50 and 70, therebymaking it possible to make the gains of the respective channels uniformto that the levels of the respective wavelengths can be madeapproximately uniform. Control for uniformalizing maximum value isperformed in a state in which the level of the light of each wavelengthis approximately uniformalized. It therefore becomes possible to performcontrol for uniformalizing the power of output light by controlling onlyone wave of the maximum power without relation to the number ofchannels. Moreover, the level (power) of each channel can be madeuniform. Further, in accordance with the sixth embodiment, it ispossible to achieve a high output and multiple channels. Furthermore, byplacing the variable light attenuator 24 between the optical-fiberamplifiers, deterioration of the S/N ratio owing to the presence of thisvariable light attenuator is mitigated and it is possible to suppress adecline in excitation efficiency.

The sixth embodiment described above relates to a case where thearrangement of FIG. 1 is used as the wavelength-division-multiplexedlight peak detector 23. However, it is also possible to adopt thearrangement of FIG. 5 in which the tunable optical filters 13 ₁-13 _(n)are cascade-connected, or the arrangement of FIG. 6 in which thelight-equalizing filter 16 is provided on the output side of the tunableoptical filter 13.

(g) Seventh Embodiment

FIG. 16 is a diagram showing the construction of a seventh embodiment ofan apparatus for controlling wavelength-division-multiplexed lightaccording to the present invention. Components in FIG. 16 identical withthose of the sixth embodiment shown in FIG. 15 are designated by likereference characters. The seventh embodiment shown in FIG. 16 differsfrom the sixth embodiment in that (1) a control loop for uniformalizingmaximum value and a control loop for uniformalizing total power areprovided as means for controlling the level of output light, and (2)control for uniformalizing maximum value and control for uniformalizingtotal power is performed based upon the value of maximum peak valueP_(peak).

As shown in FIG. 16, the apparatus includes an optical-power peakdetection circuit 91 for detecting total power P₀ of the output light(wavelength-division-multiplexed light) from the electric signal outputby the photodiode 78; a wavelength counter 92 for counting the peaks ofthe electric signal output from the photodiode 14 to thereby detect thenumber N_(ch) of multiplexed channels; and an optical-output-levelcontroller 93 for inputting a feedback signal to the excitation lightsource 29 of the optical-fiber amplifier in such a manner that (1) thedetected value of the peak becomes the set value or (2) the detectedvalue of power becomes the set value, based upon detected value ofmaximum peak (detected value P_(peak)) and the detected value P₀ ofpower.

IfP₀ >P _(peak) ·N _(ch)  (1)holds, the optical-output-level controller 93 inputs the differencebetween P_(peak)·N_(ch) and the set power P_(s) (=P_(s)−P_(peak)·N_(ch))to the variable light attenuator 24, and the latter controls the opticallevel in such a manner that the difference becomes zero. It should benoted that control for making the difference between P_(peak)·N_(ch) andthe set power P_(s) equal to zero means performing control in such amanner that the detected peak value P_(peak) becomes the set value(=P_(s)/N_(ch)). Further, ifP₀ <P _(peak) N _(ch)  (2)holds, the optical-output-level controller 93 inputs the differencebetween P₀ and the set power P_(s) (=P_(s)−P₀) to the variable lightattenuator 24, and the latter controls the optical level in such amanner that the difference becomes zero.

When Equation (1) holds, the number of channels is comparatively smalland many ASE levels (noise levels of the optical-fiber amplifier) areincluded. In such cases, even if control is performed to uniformalizethe detected value P₀ of power, the noise power contained in the lightwill be large and, as a consequence, true optical power cannot becontrolled to achieve uniformity. Accordingly, control is carried out insuch a manner that the detected peak value P_(peak) becomes the setvalue (=P_(s)/N_(ch)) When Equation (2) holds, on the other hand, thereis a possibility that the level of one channel will become excessivelylarger than the levels of other channels owing to gain tilt (gain tiltmeans that the gains of the respective wavelengths differ). In suchcase, even if control is performed so that the detected peak valueP_(peak) becomes the set value (=P_(s)/N_(ch)), the total optical powercannot be uniformalized and, moreover, the level differences betweenchannels can be made small. Accordingly, control is performed in such amanner that the detected value P₀ of power is uniformalized.

The sixth embodiment described above relates to a case where thearrangement of FIG. 1 is used as the wavelength-division-multiplexedlight peak detector 23. However, it is also possible to adopt thearrangement of FIG. 5 in which the tunable optical filters 13 ₁-13 _(n)are cascade-connected, or the arrangement of FIG. 6 in which thelight-equalizing filter 16 is provided on the output side of the tunableoptical filter 13.

In accordance with the apparatus for detecting the peaks ofwavelength-division-multiplexed light according to the presentinvention, a photodiode array and a wavelength demultiplexer are notused, as a result of which the apparatus is small in size and simple instructure. Moreover, there is no limitation upon the number of channelsand it possible to detect the maximum value of power per wave of thewavelength-division-multiplexed light (i.e., per channel). Further, byproviding periodic sweeping means, the light of each wavelength can beoutput periodically from a tunable optical filter, and it is possible toreadily detect the peak values/maximum peak value of the light of eachwavelength as well as the number of multiplexed wavelengths (number ofchannels). By cascade-connecting two or more tunable optical filters andperiodically sweeping these filters simultaneously, light of awavelength having a narrow half-width can be output. This isadvantageous in a case where wavelength spacing is small. By providing alight-equalizing filter on the output side of the tunable opticalfilter, the precision with which the peak value of each wavelength isdetected can be improved and it is possible to improve the precisionwith which peak value and number of multiplexed wavelengths aredetected.

Further, in accordance with the apparatus for controllingwavelength-division-multiplexed light according to the presentinvention, a photodiode array and a wavelength demultiplexer are notused, as a result of which the apparatus is small in size and simple instructure. Moreover, there is no limitation upon the number of channelsand it possible to detect the maximum value of power per wave of thewavelength-division-multiplexed light, and control can be carried out insuch a manner that this maximum value becomes the set value. That is, inaccordance with the present invention, it becomes possible to performcontrol to uniformalize the power of the output light by controllingonly one wave of maximum power. Further, in accordance with the presentinvention, by cascade-connecting two or more tunable optical filters andperiodically sweeping these filters simultaneously, light of awavelength having a narrow half-width can be output. Even in a casewhere wavelength spacing is small, the precision with which the opticallevel of each wavelength is detected can be improved, thereby makingpossible highly precise control for uniformalizing the level of outputlight. Further, in accordance with the present invention, by providing alight-equalizing filter on the output side of the tunable opticalfilter, the precision with which the optical power of each wavelength isdetected is improved and it is possible to perform highly precisecontrol to uniformalize the level of output light.

Further, in accordance with the apparatus for controllingwavelength-division-multiplexed light according to the presentinvention, (1) control for uniformalizing maximum value or (2) controlfor uniformalizing power is performed, based upon whether or not maximumpeak value that has been detected exceeds the level of the light of eachwavelength. Control for uniformalizing maximum value involves generatinga feedback signal in such a manner that the peak value becomes the setvalue. Control for uniformalizing power involves generating a feedbacksignal in such a manner that detected power of output light(wavelength-division-multiplexed light) becomes the set power. Thefeedback signal is input to an excitation light source of anoptical-fiber amplifier. As a result, power can be uniformalized bycontrol for uniformalizing power even in a case where the maximum peakvalue exceeds the level of the light of each wavelength. Moreover, thelevel difference of the light of each wavelength can be reduced andcontrol for uniformalizing maximum value to be carried out subsequentlycan be performed more effectively. In addition, the number ofmultiplexed wavelengths is detected based upon the number of peaks of anelectric signal output from photoelectric conversion means, and the setpower is changed in conformity with the number of multiplexedwavelengths, thereby making possible excellent control foruniformalizing optical level.

Further, in accordance with the apparatus for controllingwavelength-division-multiplexed light according to the presentinvention, (1) control for uniformalizing maximum value or (2) controlfor uniformalizing gain is performed, based upon whether or not maximumpeak value that has been detected exceeds the level of the light of eachwavelength. Control for uniformalizing maximum value involves generatinga feedback signal in such a manner that the peak value becomes the setvalue. Control for uniformalizing gain involves generating a feedbacksignal in such a manner that detected gain becomes the set gain. Thefeedback signal is input to an excitation light source of anoptical-fiber amplifier. As a result, excessive gain tilt will not becaused and control for uniformalizing maximum value can be performedeffectively. That is, if the maximum peak value becomes excessive,control to uniformalize gain is performed, the gains of the respectivechannels are made uniform and, hence, a difference in the level of thelight of each wavelength can be reduced. Control for uniformalizingmaximum value to be carried out subsequently can be performed moreeffectively.

Further, in accordance with the apparatus for controllingwavelength-division-multiplexed light according to the presentinvention, control for uniformalizing gain is performed in a gainuniformalizing controller, thereby uniformalizing the gain of eachchannel. As a result, control for uniformalizing maximum value isperformed in a state in which the level of the light of each wavelengthis approximately uniformalized. It therefore becomes possible to performcontrol for uniformalizing the power of output light by controlling onlyone wave of the maximum power without relation to the number ofchannels. Moreover, the level of each channel can be made uniform.

Further, in accordance with the present invention, it is so arrangedthat even in a WDM light control apparatus in which high output andmultiple channels can be achieved by cascade-connecting optical-fiberamplifiers, control for uniformalizing gain is performed, therebyuniformalizing the gain of each channel. As a result, control foruniformalizing maximum value is performed in a state in which the levelof the light of each wavelength is approximately uniformalized. It ispossible, therefore, to perform control for uniformalizing the power ofoutput light by controlling only one wave of the maximum power withoutrelation to the number of channels. Moreover, the level of each channelcan be made uniform. Furthermore, it is possible to achieve a highoutput and multiple channels. By placing a variable light attenuatorbetween optical-fiber amplifiers, deterioration of the S/N ratio owingto the presence of this variable light attenuator is mitigated and it ispossible to suppress a decline in excitation efficiency.

As many apparently widely different embodiments of the present inventioncan be made without departing from the spirit and scope thereof, it isto be understood that the invention is not limited to the specificembodiments thereof except as defined in the appended claims.

1. An apparatus for detecting peaks of wavelength-division-multiplexedlight, comprising: a tunable optical filter having a bandwidth toselectively pass light of each wavelength ofwavelength-division-multiplexed light; periodic sweeping means forperiodically outputting light of each wavelength from the tunableoptical filter; and peak detection means for detecting the peaks of thelight output from the tunable optical filter, wherein one or more othertunable optical filters are cascade-connected to the tunable opticalfilter, and each of the cascade-connected tunable optical filters areperiodically swept synchronously.
 2. An apparatus for controllingwavelength-division-multiplexed light, comprising: optical level controlmeans for controlling an optical level ofwavelength-division-multiplexed light; optical branching means forbranching a portion of wavelength-division-multiplexed light output fromthe optical level control means; a tunable optical filter to selectivelyoutput light of each wavelength of the branchedwavelength-division-multiplexed light; peak detection means fordetecting peaks of the light output from the tunable optical filter; andfeedback means for inputting a feedback signal to the optical levelcontrol means so that a maximum peak value will become a set value,wherein one or more other tunable optical filters are cascade-connectedto the tunable optical filter, and each of the cascade connected tunableoptical filters are periodically swept synchronously.
 3. An apparatus tocontrol wavelength-division-multiplexed light, comprising: an opticalamplifier to amplify wavelength-division-multiplexed light; a tunableoptical filter to selectively output light of each wavelength of aportion of the wavelength-division-multiplexed light; peak detectionmeans for detecting peaks of the light from said tunable optical filter;power detection means for detecting total power ofwavelength-division-multiplexed light output from the optical amplifier;and feedback means for generating a feedback signal depending on themaximum peak value, so that a maximum peak value will become a setvalue, or so that the detected power will become a set power, and forinputting the feedback signal to an excitation light source of theoptical amplifier.
 4. The apparatus according to claim 3, furthercomprising: power detection means for detecting a total power ofwavelength-division-multiplexed light output from the tunable opticalfilter; and photoelectric conversion means for photoelectricallyconverting light output from the tunable optical filter and detectingmeans for detecting number of multiplexed wavelengths based upon numberof peaks of the electric signal output from the photoelectric conversionmeans, wherein, depending on the maximum peak value, the feedback meansinputs a feedback signal to an excitation light source of the opticalamplifier such that one of the maximum peak value becomes the set valueand the detected power becomes a set power, and wherein the set power ischanged in conformity with the number of multiplexed wavelengths.
 5. Theapparatus according to claim 3, wherein one or more other tunableoptical filters are cascade-connected to the tunable optical filter, andeach of the one or more tunable optical filters are periodically sweptsynchronously.
 6. The apparatus according to claim 3, further comprisinga light-equalizing filter located with the tunable optical filter. 7.The apparatus according to claim 3, wherein the feedback means generatesthe feedback signal so that the detected power will become the set powerwhen the maximum peak value exceeds a threshold in comparison with thepeak value of the light of the other wavelengths, and the feedback meansgenerates the feedback signal so that the maximum peak value will becomethe set value when the maximum peak value exceeds a threshold incomparison with the peak value of the light of the other wavelengths. 8.An apparatus for detecting peaks of wavelength-division-multiplexedlight, comprising: a tunable optical filter having a bandwidth toselectively pass light of each wavelength ofwavelength-division-multiplexed light; a periodic sweeping unit toperiodically output light of each wavelength from the tunable opticalfilter; and peak detection unit to detect peaks of the light output fromthe tunable optical filter, wherein one or more other tunable opticalfilters are cascade-connected to the tunable optical filter, and each ofthe cascade-connected tunable optical filters are periodically sweptsynchronously.
 9. An apparatus for controllingwavelength-division-multiplexed light, comprising: optical level controlunit to control an optical level of wavelength-division-multiplexedlight; optical branching unit to branch a portion of thewavelength-division-multiplexed light output from the optical levelcontrol unit; a tunable optical filter to selectively output light ofeach wavelength of the branched wavelength-division-multiplexed light;peak detection unit to detect peaks of the light output from the tunableoptical filter; and a feedback unit to input a feedback signal to theoptical level control unit so that maximum peak value will become a setvalue, wherein one or more other tunable optical filters arecascade-connected to said tunable optical filter, and each of thecascade-connected tunable optical filters are periodically sweptsynchronously.
 10. An apparatus for controllingwavelength-division-multiplexed light, comprising: an optical amplifierto amplify wavelength-division-multiplexed light; a tunable opticalfilter to selectively output light of each wavelength of a portion ofthe wavelength-division-multiplexed light; peak detection unit to detectpeaks of the light from the tunable optical filter; a power detectionunit to detect total power of wavelength-division-multiplexed lightoutput from the optical amplifier; and a feedback unit to generate afeedback signal depending on the maximum peak value, so that a maximumpeak value will become a set value, or so that the detected power willbecome a set power, and to input the feedback signal to an excitationlight source of the optical amplifier.